Bluff Body Drag Reduction Via Venting
Bluff bodies, such as spheres, are generally characterized by high drag coefficients when exposed to an airflow. The high drag coefficients are caused by early separation of a boundary layer, resulting in a large unsteady wake that is dominated by large eddies and low average pressure. Research has shown that the unsteadiness of the wake is responsible for large amounts of kinetic energy being continually shed in the wake (von Karman1, Roshko2), which causes high drag.
Blowing of the wake region has been shown to be effective in reducing drag coefficient (Cd) on 2D and 3D bodies (Bearman3, Wood4, etc.) Normal blowing in the wake region is referred to as base bleed and has been shown effective in reducing the values of drag coefficient for bluff bodies. However, the external provision of fluid for base bleed is energetically expensive and it increases the level of complexity of the solution.
A simpler counterpart to powered base bleed is the passive concept of wake blowing via vented bodies. A duct connects the leading edge to the trailing edge regions, allowing the natural pressure difference to induce a flow in the duct. This duct flow has been shown to influence the wake and reduce drag.
Vented 2D bodies have been proposed and investigated in the past, and results of such investigations are published in the literature.
A vented sphere concept (FIG. 1) has been proposed and investigated by Meier, Suryanarayana, and Pauer5 in 1990. The concept has been further investigated by Suryanarayana, Pauer, and Meier6 in 1993, and by Suryanarayana and Prabhu7 in 2000. For the configurations investigated the duct diameter was equal to 15% of the diameter of the sphere, resulting in a duct cross sectional area of only 2.25% of the projected area of the sphere. Their published results showed up to 60% reduction in Cd in the supercritical Re number regime.
Recent Computational Fluid Dynamics (CFD) investigations by the inventor provided results that correlate well with the experimental results discussed in the previous paragraph. They confirm the flow characteristics observed and reported in the experimental investigation, and suggest that the major mechanism for drag reduction is based upon the steadying effect that the duct flow has over the wake flow. A pair of stationary, counter-rotating vortex rings form on the leeward side and provide a boat-tailing effect for the flow (FIG. 2).
Bluff Body Airloads Near a Wall
Fluid flow over a body can be significantly influenced by the proximity of a wall or other surface, resulting in aerodynamic coefficients that can be significantly different from those in free flight.
As an example, Computational Fluid Dynamics (CFD) calculations at Reynolds (Re) numbers on the order of 106 show that in free flight the drag coefficient of a sphere (a type of bluff body of interest for numerous practical applications) is Cd=0.2 and the lift coefficient is Cl=0. At the same Re number for a sphere tangent to a ground plane (FIG. 3) CFD calculations show that the drag and lift coefficients become Cd=0.76 and Cl=0.56, respectively. The increased aerodynamic coefficients are responsible for corresponding increases in airloads.
In the case of a body that is moored to the ground such an increase in airloads requires corresponding increases in mooring loads, which is undesirable. In the case of a sphere tangent to a ground plane, the resulting drag is increased and a positive lift that tends to move the sphere away from the ground plane also occurs (FIG. 4).
From a physical perspective, the increased airloads are caused by the flow blockage in the region where the sphere is tangent to the ground plane (FIG. 3), resulting in a pressure distribution as shown in FIG. 4.
Airship Hull Shapes
Airships generate static lift through buoyancy, which is proportional with the volume occupied by the lifting gas contained in the hull of the airship. The static lift is equal to the buoyancy force minus the weight of the airship, a significant component of which is given by the weight of the envelope. This in turn is proportional to the surface area of the envelope.
It follows that in order to maximize the lift produced by a given volume of lifting gas, the surface area of the envelope should be minimized. It is well known that a sphere minimizes the surface area for a given volume. Therefore it would seem that low aspect ratio, and in the limit spherical envelope airships, should be desirable. However, a simple inspection of classical airship shapes shows this not to be the case for at least the reasons that (1) Low aspect ratio, and in the limit spherical hulls are bluff bodies characterized by high drag; and (2) Low aspect ratio hulls are aerodynamically unstable, having a tendency to broadside.
Conventional airship hulls have elongated shapes, typically with an aspect ratio (length divided by maximum transverse diameter) higher than 3. However, while characterized by lower drag and lower level of aerodynamic instability they are nevertheless affected by deficiencies of their own, which include the following:                Weight penalty associated with much larger envelope area;        Additional weight penalty due to increased envelope material strength requirements due to increased structural loads on the elongated airship hull;        Limited turn performance, resulting in large airship turn radii;        Large required ground footprint for mooring in order to allow an airship moored by the nose of the hull to freely weathervane with the wind;        Lifting gas sloshing instability at low fill factors, which can cause the airship hull to uncontrollably reach a fully nose up or fully tail up attitude.        
A class of toroidal shape hull airships has been proposed. In 1986 Todd has proposed the Toroidal Balloon Concept shown in FIG. 5. In 1991 Tachibana et al. have proposed the Toroidal Semi-Buoyant Station shown in FIG. 6. Both of these concepts represent airship configurations for which the buoyancy of the lifting gas is simply used to supplement the lift from a main rotor. This essentially is a helicopter-balloon hybrid. The axis of the hull duct is oriented vertically and the configuration is not proposed or intended for hull drag reduction in forward flight.
Technologies that are capable of significant hull drag reduction and/or of alleviating the aerodynamic hull instabilities have the potential to enable the development of low hull aspect ratio airships and are desirable.
Airship Mooring
Conventional airships are typically moored to a ground mast by the nose of the hull and are allowed to weathervane around the mast with the wind. Consequently, the required obstacle clear mooring area is a circle of very large radius, larger than the length of the airship hull. This represents a significant problem, in particular for airships that need to be operated from unimproved, tactically selected locations.
For spherical and other unconventional airships, a mooring method that positions and restrains the airship tangent to the ground has been used. However, as discussed above, this mooring method results in very high wind airloads which can lead to structural failures.